Respiration consists of two critical and nonoverlapping processes, ventilation and diffusion. Ventilation is the bulk movement of gases in and out of the lungs via the large and intermediate airways that collectively are termed the conducting zone. Here, inspired gas is warmed to 37°C, humidified to 100% relative humidity (RH), and cleansed of most airborne particulates, but its molecular composition is changed only by addition of water vapor. This is because conducting zone airways are too wide and their walls are too thick to allow appreciable molecular diffusion of O₂ and CO₂ between inspired air and the pulmonary arterial blood coursing through alveolar capillaries. On the other hand, the delicate alveolar membranes of the respiratory parenchyma are thin and have an enormous aggregate surface area. As will be formally introduced in Chap. 9, molecular diffusion requires such large, thin surfaces to maximize O₂ and CO₂ exchange. The remainder of this chapter will focus on ventilation, and the forces and constraints that make it more or less effective. It is worth noting now that the conducting zone volume is nearly constant during normal breathing, while respiratory parenchymal volume changes very dramatically as the depth of breathing is adjusted.

Despite the histological complexity of lung tissue (Chap. 2), its key elements can be represented by a simple line drawing (Fig. 4.1). The many branching airways of the conducting zone that support ventilation are shown as a single tube comprising the anatomic dead space (V_D) (Fig. 4.2). V_D is normally ~2 mL/kg of ideal body weight, or about 160 mL in a normal adult. In Fig. 4.1, the respiratory parenchyma where diffusion occurs is shown as one contiguous compartment, the alveolar gas volume (V_A), here 3.0 L. We define tidal volume (V_T) as the total volume of a typical inspiration, but clearly only a portion of each breath reaches the parenchyma while the remainder occupies V_D. Thus, in this example where V_D = 160 mL, only 440 mL of a 600 mL V_T reaches alveoli of the respiratory parenchyma, or 740 mL of a 900 mL V_T. Further, we define total ventilation, V̇_E (L/min) as the product of V_T and respiratory frequency, f (breaths/min). Using the data of Fig. 4.1, V̇_E = 9.0 L/min (= 0.60 L/breath × 15 breaths/min). Consequently, alveolar ventilation, V̇_A (L/min) equals (V_T – V_D) × f. Using volumes in Fig. 4.1, we can compute that at a resting f = 15 breaths/min and V_T = 0.60 L, the resulting V̇_A of 6.6 L/min will exchange a V_A of 3.0 L about twice each minute.

Based on lung tissue morphometry, alveolar capillary volume (V_C) contains perhaps 70-100 mL of blood at any instant. Since baseline cardiac output (Q̇) is ~6 L/min, a subject’s V_C is typically replenished with mixed venous blood via the pulmonary arterioles at least once per second at rest [(6,000 mL/min)/70 mL] and more frequently during exercise. Thus, erythrocytes spend less than a second traversing a pulmonary capillary that is sufficiently close to alveolar air for diffusive gas exchange. We can also estimate that for an ideal lung with homogeneous architecture from apex to base, the alveolar ventilation/perfusion ratio, V̇_A/Q̇ is ~1.0 (ie, 6.6 L/6.0 L). Whether an ideal V̇_A/Q̇ ratio of 1.0 actually occurs in normal lung will be discussed in Chap. 8.

Most lung volumes are measured directly by spirometry, an apparatus originally consisting of an inverted, water-sealed bell filled with 100% O₂ from which a subject breathes through a mouthpiece while wearing nose clips (Fig. 4.3). As gases move in and out of the bell, pen tracings record volume changes even as expired CO₂ is trapped chemically by an internal canister of soda lime. Thus, the eventual decline in bell height (superimposed on ventilatory pen movements) reflects V̇o₂. Routine spirometry provides multiple lung volumes plus the rate functions V̇_E and V̇o₂.

Most spirometric volumes are self-explanatory. A capacity is the sum of two or more volumes. After a normal expiration, the amount of gas remaining in the lungs is called the functional residual capacity (FRC), an important reference value and the standard starting point for other pulmonary function measurements. By forcefully exhaling as much gas as possible beyond a normal V_T, we see that FRC is really the sum of the expiratory reserve volume (ERV)

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